Tuning Magnetic and Structural Transitions through Valence Electron Concentration in the Giant Magnetocaloric Gd_5–<i>x</i>Eu_<i>x</i>Ge₄ Phases

Valence electron concentration is a viable chemical tool to control the crystal structure and magnetism of Gd₅Ge₄. A decrease in the valence electron concentration achieved through the substitution of Eu²⁺ for Gd³⁺ leads to the formation of the interslab Ge–Ge dimers, phase transitions to the Gd₅Si₂Ge₂- and Gd₅Si₄-type structures, and a ferromagnetic ordering in the Gd_5–<i>x</i>Eu_<i>x</i>Ge₄ system. Gd_4.75Eu_0.25Ge₄ and Gd_4.50Eu_0.50Ge₄ undergo temperature-induced magnetostructural transformations accompanied by giant magnetocaloric effects

Electron-Deficient Eu_6.5Gd_0.5Ge₆ Intermetallic: A Layered Intergrowth Phase of the Gd₅Si₄- and FeB-Type Structures

A novel electron-poor Eu_6.5Gd_0.5Ge₆ compound adopts the Ca₇Sn₆-type structure (space group <i>Pnma</i>, <i>Z</i> = 4, <i>a</i> = 7.5943(5) Å, <i>b</i> = 22.905(1) Å, <i>c</i> = 8.3610(4) Å, and <i>V</i> = 1454.4(1) ų). The compound can be seen as an intergrowth of the Gd₅Si₄-type (<i>Pnma</i>) R₅Ge₄ (R = rare earth) and FeB-type (<i>Pnma</i>) RGe compounds. The phase analysis suggests that the Eu_7–<i>x</i>Gd_<i>x</i>Ge₆ series displays a narrow homogneity range of stabilizing the Ca₇Sn₆ structure at <i>x</i> ≈ 0.5. The structural results illustrate the structural rigidity of the _∝²[R₅X₄] slabs (X = <i>p</i>-element) and a possibility for discovering new intermetallics by combining the _∝²[R₅X₄] slabs with other symmetry-approximate building blocks. Electronic structure analysis suggests that the stability and composition of Eu_6.5Gd_0.5Ge₆ represents a compromise between the valence electron concentration, bonding, and existence of the neighboring EuGe and (Eu,Gd)₅Ge₄ phases

Tuning Magnetic and Structural Transitions through Valence Electron Concentration in the Giant Magnetocaloric Gd_5–<i>x</i>Eu_<i>x</i>Ge₄ Phases

Valence electron concentration is a viable chemical tool to control the crystal structure and magnetism of Gd₅Ge₄. A decrease in the valence electron concentration achieved through the substitution of Eu²⁺ for Gd³⁺ leads to the formation of the interslab Ge–Ge dimers, phase transitions to the Gd₅Si₂Ge₂- and Gd₅Si₄-type structures, and a ferromagnetic ordering in the Gd_5–<i>x</i>Eu_<i>x</i>Ge₄ system. Gd_4.75Eu_0.25Ge₄ and Gd_4.50Eu_0.50Ge₄ undergo temperature-induced magnetostructural transformations accompanied by giant magnetocaloric effects

Synthetic Approach for (Mn,Fe)₂(Si,P) Magnetocaloric Materials: Purity, Structural, Magnetic, and Magnetocaloric Properties

A conventional solid-state approach has been developed for the synthesis of phase-pure magnetocaloric Mn_2–<i>x</i>Fe_<i>x</i>Si_0.5P_0.5 materials (<i>x</i> = 0.6, 0.7, 0.8, 0.9). Annealing at high temperatures followed by dwelling at lower temperatures is essential to obtain pure samples with <i>x</i> = 0.7, 0.8, and 0.9. Structural features of the samples with <i>x</i> = 0.6 and 0.9 were analyzed as a function of temperature via synchrotron powder diffraction. The Curie temperature, temperature hysteresis, and magnetic entropy change were established from the magnetic measurements. According to the diffraction and magnetization data, all samples undergo a first-order magnetostructural transition, but the first-order nature becomes less pronounced for samples that are more Mn rich

Crystal Cluster Growth and Physical Properties of the EuSbSe₃ and EuBiSe₃ Phases

Syntheses of europium metal, selenium powder, and the Sb₂Se₃/Bi₂Se₃ binaries were observed to produce crystal clusters of the EuSbSe₃ and EuBiSe₃ phases. These phases crystallize with the <i>P</i>2₁2₁2₁ space group and can be easily identified based on their growth habits, forming large clusters of needles. Previous literature suggested that their structure is charge-balanced with all europium atoms in the divalent state and one-quarter of the selenium atoms forming trimers. Physical property measurements on a pure sample of EuSbSe₃ revealed typical Arrhenius-type electrical resistivity, being approximately 3 orders of magnitude too large for thermoelectric applications. Electronic structure calculations indicated that both EuSbSe₃ and EuBiSe₃ are narrow-band-gap semiconductors, in good agreement with the electrical resistivity data. The valence and conduction band states near the Fermi level are dominated by the Sb/Bi and Se p states, as expected given their small difference in electronegativity

AlFe_2–<i>x</i>Co_<i>x</i>B₂ (<i>x</i> = 0–0.30): <i>T</i>_C Tuning through Co Substitution for a Promising Magnetocaloric Material Realized by Spark Plasma Sintering

AlFe₂B₂ and AlFe_2–<i>x</i>Co_<i>x</i>B₂ (<i>x</i> = 0–0.30) were synthesized from the elements in three different ways. The samples were characterized by powder X-ray diffraction, Rietveld refinements, energy-dispersive X-ray spectroscopy, and magnetic measurements. Using Al flux the formation of AlFe₂B₂ single crystals is preferred. Arc melting enables the substitution of ∼6% Co. This substitution of Fe by Co decreases the Curie temperature <i>T</i>_C from 290 to 240 K. The highest Co substitution up to 15% is achieved by spark plasma sintering (SPS). <i>T</i>_C is reduced to 205 K. In all cases an excess of Al is necessary to avoid the formation of ferromagnetic FeB. Al₁₃Fe_4–<i>x</i>Co_<i>x</i> is the common byproduct. <i>T</i>_C and the cobalt content are linearly correlated. The transition paramagnetic–ferromagnetic remains sharp for all examples. The magnetic entropy change of the Co-containing samples is comparable to AlFe₂B₂. SPS synthesis yields, in short reaction times, a homogeneous and dense material with small amounts of paramagnetic Al₁₃Fe_4–xCo_<i>x</i> as an impurity, which can serve as sinter additive. These properties make AlFe_2–<i>x</i>Co_<i>x</i>B₂ a promising magnetocaloric material for applications between room temperature and 200 K

Crystal Cluster Growth and Physical Properties of the EuSbSe₃ and EuBiSe₃ Phases

Syntheses of europium metal, selenium powder, and the Sb₂Se₃/Bi₂Se₃ binaries were observed to produce crystal clusters of the EuSbSe₃ and EuBiSe₃ phases. These phases crystallize with the <i>P</i>2₁2₁2₁ space group and can be easily identified based on their growth habits, forming large clusters of needles. Previous literature suggested that their structure is charge-balanced with all europium atoms in the divalent state and one-quarter of the selenium atoms forming trimers. Physical property measurements on a pure sample of EuSbSe₃ revealed typical Arrhenius-type electrical resistivity, being approximately 3 orders of magnitude too large for thermoelectric applications. Electronic structure calculations indicated that both EuSbSe₃ and EuBiSe₃ are narrow-band-gap semiconductors, in good agreement with the electrical resistivity data. The valence and conduction band states near the Fermi level are dominated by the Sb/Bi and Se p states, as expected given their small difference in electronegativity

Gd₄Ge_3–<i>x</i><i>Pn</i>_<i>x</i> (<i>Pn</i> = P, Sb, Bi, <i>x</i> = 0.5–3): Stabilizing the Nonexisting Gd₄Ge₃ Binary through Valence Electron Concentration. Electronic and Magnetic Properties of Gd₄Ge_3–<i>x</i><i>Pn</i>_<i>x</i>

Gd₄Ge_3–<i>x</i><i>Pn</i>_<i>x</i> (<i>Pn</i> = P, Sb, Bi; <i>x</i> = 0.5–3) phases have been prepared and characterized using X-ray diffraction, wavelength-dispersive spectroscopy, and magnetization measurements. All Gd₄Ge_3–<i>x</i><i>Pn</i>_<i>x</i> phases adopt a cubic anti-Th₃P₄ structure, and no deficiency on the Gd or <i>p</i>-element site could be detected. Only one P-containing phase with the Gd₄Ge_2.51(5)P_0.49(5) composition could be obtained, as larger substitution levels did not yield the phase. Existence of Gd₄Ge_2.51(5)P_0.49(5) and Gd₄Ge_2.49(3)Bi_0.51(3) suggests that the hypothetical Gd₄Ge₃ binary can be easily stabilized by a small increase in the valence electron count and that the size of the <i>p</i> element is not a key factor. Electronic structure calculations reveal that large substitution levels with more electron-rich Sb and Bi are possible for charge-balanced (Gd³⁺)₄(Ge^4–)₃ as extra electrons occupy the bonding Gd–Gd and Gd–Ge states. This analysis also supports the stability of Gd₄Sb₃ and Gd₄Bi₃. All Gd₄Ge_3–<i>x</i><i>Pn</i>_<i>x</i> phases order ferromagnetically with relatively high Curie temperatures of 234–356 K. The variation in the Curie temperatures of the Gd₄Ge_3–<i>x</i>Sb_<i>x</i> and Gd₄Ge_3–<i>x</i>Bi_<i>x</i> series can be explained through the changes in the numbers of conduction electrons associated with Ge/Sb­(Bi) substitution

Disorder-Controlled Electrical Properties in the Ho₂Sb_1–<i>x</i>Bi_<i>x</i>O₂ Systems

High-purity bulk samples of the Ho₂­Sb_1–<i>x</i>­Bi_<i>x</i>O₂ phases (<i>x</i> = 0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared and subjected to structural and elemental analysis as well as physical property measurements. The Sb/Bi ratio in the Ho₂­Sb_1–<i>x</i>­Bi_<i>x</i>O₂ system could be fully traversed without disturbing the overall <i>anti</i>-Th­Cr₂Si₂ type structure (<i>I</i>4/<i>mmm</i>). The single-crystal X-ray diffraction studies revealed that the local atomic displacement on the Sb/Bi site is reduced with the increasing Bi content. Such local structural perturbations lead to a gradual semiconductor-to-metal transition in the bulk materials. The significant variations in the electrical properties without a change in the charge carrier concentration are explained within the frame of the disorder-induced Anderson localization. These experimental observations demonstrated an alternative strategy for electrical properties manipulations through the control of the local atomic disorder

Decoupling the Electrical Conductivity and Seebeck Coefficient in the <i>RE</i>₂SbO₂ Compounds through Local Structural Perturbations

Compromise between the electrical conductivity and Seebeck coefficient limits the efficiency of chemical doping in the thermoelectric research. An alternative strategy, involving the control of a local crystal structure, is demonstrated to improve the thermoelectric performance in the <i>RE</i>₂SbO₂ system. The <i>RE</i>₂SbO₂ phases, adopting a disordered <i>anti</i>-ThCr₂Si₂-type structure (<i>I</i>4/<i>mmm</i>), were prepared for <i>RE</i> = La, Nd, Sm, Gd, Ho, and Er. By traversing the rare earth series, the lattice parameters of the <i>RE</i>₂SbO₂ phases are gradually reduced, thus increasing chemical pressure on the Sb environment. As the Sb displacements are perturbed, different charge carrier activation mechanisms dominate the transport properties of these compounds. As a result, the electrical conductivity and Seebeck coefficient are improved simultaneously, while the number of charge carriers in the series remains constant